The purpose of this study is to correlate the sequential changes in ultrastructural morphology of the developing glomerulus in the newly born rat with histochemical, functional, and immunologic characteristics. The study will stress a) the development of fixed negative charges in the capillary walls; b) the permeability of the capillaries of various stages of maturity to neutral, anionic, and cationic dextran molecules of relatively small and large molecular weight; c) the development of the mesengial cell system and the relationship of these cells to the juxtaglomerular system; d) the function of the efferent pathway for clearance of macromolecules from the mesangium to extra-glomerular sites, and; e) will search for receptors for compliment (C3b) in the developing rat glomerulus. The approach will involve micro injection of newborn rats with various dextran species and lysozyme, electron microscopy, histochemistry, immunofluorescence, and quantitative morphometric techniques.